Until well into the 1960s there was a a fairly close connection between one's degree of education and/or one's social class and one's accent
I hope I haven't misunderstood. What do you mean here by 'degree of education'?
I meant, roughly speaking that a person who speaks as she does is unlikely, to have left school at sixteen.
I'm not entirely sure I follow the thought. Why do you think somebody who speaks like her is unlikely to have left school at sixteen? Because people of her class were likely to have been encouraged to go to university?
Are you attributing the possibility of her having gone to university as in some way playing a
causal role in the development of her accent/manner of speech? This may be where I've misunderstood. If the connection between somebody's having gone to university and their accent or manner of speech is merely statistical (i.e. non-causal), in what way is it relevant?
'Accent' was the wrong word - I was thinking of her manner of delivery, sentence structure, vocabulary and accent.
All of which you mean are signs of being 'fairly educated'? Again, it's not clear to me in what sense you're using the word 'educated'. Do you mean in the sense of having been taught how to speak and act in a certain way that might be identified as normative to her social class?
It seems to me that it has sometimes historically been used euphemistically and derogatorily by the upper/middle classes as a means of class identification, for whatever end.
I'm sure it has. If you can think of a better term, please share it.
Like I've said, I suspect you may in fact be alluding to signs of class rather than to education. The fact that up until the 1960s people of a higher social class are more likely to have gone to university is not relevant, even if the case remains today. It strikes me that the reason she speaks like that is likely to have very much to do with her social background, and very little, if anything at all, to do with her education.
I wouldn't rule out, however, the possibility that since she is an actress by profession, she may have undergone some training in how to speak in such a way. Still, I don't think that would count as what people would generally understand by the word 'educated'.